Roughly speaking, uniform heating inside the heating compartment of a microwave oven is achieved by the use of a turntable, stirrer, or antenna. Now, a brief description will be given of how uniform heating is achieved by the use of each of these. Where a turntable is used, a heating target is placed on the turntable provided on the floor of the heating compartment, and the turntable is rotated. Thus, the high-frequency wave radiated into the heating compartment through an opening formed on a side wall surface or the ceiling surface thereof strikes the heating target uniformly from all directions, thereby heating it. This is the method that is currently most commonly used in microwave ovens. FIGS. 32 and 33 are a sectional view and a perspective view, respectively, of an example of a microwave oven adopting this method. A motor 5 is provided on the outside of the floor of the heating compartment 1, and the spindle 51 of this motor 5 penetrates the floor of the heating compartment 1 through a hole formed thereon so as to protrude inward from the floor of the heating compartment 1. On this spindle 51 protruding from the floor of the heating compartment 1, a disk-shaped turntable T is pivoted so that, as the motor 5 is driven, the turntable T rotates. On the other hand, the high-frequency wave radiated from a magnetron (high-frequency generator) 2 is guided through a waveguide 33, and is then radiated into the heating compartment 1 through an opening 101 formed on the side surface of the heating compartment 1. Thus, the high-frequency wave strikes the heating target (not illustrated) placed on the rotating turntable T, thereby heating it. The turntable T may instead be driven with magnetic coupling as disclosed in, for example, Japanese Patent Application Published No. S61-13359 and Japanese Patent Applications Laid-Open Nos. S58-220387 and S59-14294.
Where a stirrer is used, typically, as shown in FIGS. 34 and 35, a metal high-frequency wave diffusing wheel is provided as the stirrer in this case, close to the heating compartment 1, inside an opening 101 formed in the ceiling surface of the heating compartment 1. This wheel is rotated with a motor 31 so that, as the high-frequency wave radiated from a magnetron 2 is radiated into the heating compartment 1 through the opening 101, the intensity of the electromagnetic field of the high-frequency wave is varied by diffusion by the rotating wheel. With this method, uniform heating is possible with no movement in the heating target. The heating target is placed on a stage T′ substantially rectangular in shape and made of a dielectric material (typically glass, ceramic, or the like).
Where an antenna is used, for example as shown in FIG. 36, while, on one hand, the high-frequency wave radiated from a magnetron 2 is guided through a waveguide 3 to the outside of the floor of the heating compartment 1, the receiver portion 41 of an antenna 4 is put through an opening 11 formed on the floor of the heating compartment 1 so as to protrude into the waveguide 3 so that, on the other hand, the high-frequency wave inside the waveguide 3 is propagated from the receiver portion 41 to the radiator portion 42 of the antenna 4. This radiator portion 42 of the antenna 4 is rotated with a motor 5 so that the high-frequency wave heats the heating target uniformly (for example, as disclosed in Japanese Patent Application Laid-Open No. H11-8057). The heating target is placed on a stage 6 that is provided above and close to the antenna so as to partition the interior of the heating compartment 1 and that is made of a dielectric material (typically glass, ceramic, or the like). This method permits the heating target to be placed near the radiator portion 42 of the antenna 4 from which the high-frequency wave is radiated, and is thus superior to the other methods in heating efficiency. Today, this method is becoming increasingly widespread in microwave ovens for use in convenience stores and other food processing and selling businesses.
Of these different methods for uniform heating of a heating target, whereas that using a turntable keeps the heating target rotating while it is heated, that using a stirrer and that using an antenna keep it at rest while it is heated. From the viewpoint of uniform heating, it is generally believed that the methods using a turntable, an antenna, and a stirrer are the best, second-best, and third-best, respectively.
From the viewpoint of the area inside the heating compartment which can be used for the placement of the heating target, however, whereas the method using a turntable only offers the area of the turntable itself, that using a stirrer and that using an antenna, which require no movement in the heating target, offer the whole area of the floor of the heating compartment. Thus, the latter two permit more efficient use of the heating compartment, and accordingly permit more of the heating target to be heated at a time, provided that the volume of the heating compartment is equal.
From the viewpoint of easy cleaning of the floor of the heating compartment, the method using a turntable with magnetic coupling and that using a stirrer does not need through holes formed on the floor of the heating compartment, and thus permits comparatively easy cleaning of the floor of the heating compartment because this surface is largely flat once the turntable or stage is removed. Also with the method using an antenna, the stage provided fixedly above the antenna virtually serves as the floor wall of the heating compartment, and the surface of this stage is extremely easy to clean because it is not only flat but also made of a dielectric material such as glass or ceramic.
In recent years, increasingly high importance has come to be placed on heating efficiency, efficient use of the interior volume of the heating compartment, and easy cleaning of the heating compartment. This trend has been accompanied by revaluation of antenna-type microwave ovens for household use.
Incidentally, some recently developed microwave ovens are given composite functions by being equipped for, as well as heating using a high-frequency wave, grill heating and oven heating using a heater. Grill heating is achieved by the use of a glass-tube heater or sheath heater provided on the ceiling of the heating compartment, off the center thereof. With this heater heated so that its surface temperature is 600° C. or higher, the heating target is rotated so that it as a whole is roasted uniformly and quickly.
On the other hand, in a conventional antenna-type microwave oven, the heating target remains at rest on the stage, and therefore, to give the microwave oven composite functions, for example by adding thereto a capability of grill heating, a heater H needs to be arranged over the entire ceiling surface of the heating compartment 1. Arranging the heater H over the entire ceiling surface, however, results in the heater H occupying a large area. This lowers the temperature to which the heater H can be heated, and increases the duration for which it needs to be heated. Disadvantageously, the heating duration cannot be shortened without increasing the power consumption by the heater.
Moreover, the method using an antenna, just because it keeps the heating target at rest, occasionally produces unsatisfactory results in the preparation of, for example, egg dishes such as chawan-mushi, a Japanese egg-based pot-steamed hotchpotch, which require delicately controlled uniform heating.
Other modern microwave ovens are equipped with stirring foodstuffs. With these, the entire procedure for preparing a dish, for example a stewed dish such as curried stew, or for preparing dough for bread can be gone through continuously, from the preparation of ingredients up to the heating and finishing of the target dish. An example of this type of microwave oven is shown in FIG. 38. The microwave oven shown in FIG. 38 has a turntable, which can be interchanged with a container 8 having a stirring wheel 83 inside it. When this container 8 is placed inside the heating compartment 1, the rotary shaft 82 of the stirring wheel 83 is coupled with the spindle 51 of the motor 5 for rotating the turntable. As the stirring wheel 83 is rotated inside the container 8, it stirs the foodstuffs put therein (for example, as disclosed in Japanese Patent Applications Laid-Open Nos. H10-211098 and H11-121161).
On the other hand, in a conventional antenna-type microwave oven, there is provided no mechanism for driving a stirring wheel. This makes it impossible to add thereto a function of automatic stirring.